1. Field of the Invention
The present invention relates generally to a booster-compressor for a turboprop aircraft engine and particularly concerns an asymmetric turboprop booster and a propeller driven through a gearbox by a gas turbine engine. The booster is provided with an asymmetric inlet and outlet and specially designed rotors and stators which maintain a substantially constant static pressure along streamlines from the booster inlet to the booster outlet.
2. Discussion of the Background Art
In order to increase the thrust developed by conventional gas turbine jet aircraft engines, supercharged air can be driven into the inlet of the core engine, also known as the gas generator, with a booster type compression system. Such a system typically includes several rows of rotor blades and several rows of co-acting stator vanes which raise the pressure of the air entering the core engine allowing it to develop more power while minimizing engine size and weight.
A turboprop engine is generally defined as an engine which does not have a duct surrounding its propeller blades which are typically driven by a turbine through a gearbox and are variable pitch as contrasted with a turbofan engine which is typically defined as an engine which includes a duct surrounding its rotating fan blades and which are typically directly driven by a turbine with no gearbox and are fixed pitch. Because of the differences in operation and design between turbofan engines and turboprop engines, conventional booster designs suitable for use with turbofan engines do not appear to offer significant improvements in the thrust developed by turboprop engines.
Accordingly, a need existed for a booster for supercharging inlet air into the core engine of a turboprop engine using a minimum number of booster stages and a minimum number of blades and vanes. Ideally, a single stage booster having a single row of rotor blades and a single row of stator vanes would produce a significant increase in the pressure of the air introduced into the core engine of a turboprop aircraft engine.
A single stage booster-compressor designed for use with a turboprop aircraft engine that meets this need was developed and disclosed in U.S. Pat. No. 5,345,760 which is incorporated herein by reference. This booster-compressor has a symmetric booster inlet and directs only a portion of its supercharged air into a core inlet of a core engine gas generator of a turboprop engine. The symmetric annular flowpath of the booster-compressor is split into two separate flowstreams at the booster outlet. One flowstream enters the core engine while the other exits the booster-compressor through an exhaust nozzle leading directly into the ambient atmosphere.
The flow which directly enters the atmosphere through the booster exit exhaust nozzle provides supplemental thrust while allowing the cross-sectional area of the flowpath of the booster-compressor to be increased over that area which would be required for providing supercharged air only to the core engine. The increase in flowpath section provided by bypassing a portion of the supercharged air to ambient allows the rotor blades and stator vanes of the booster-compressor to be sized for low blade and vane count and for ease of manufacture and assembly.
This design has a few drawbacks. The Mach number of the flow leaving the booster nozzle is necessarily higher than without the booster because of the pressure input by the booster. This higher Mach number flow scrubs the gas generator nacelle, creates additional drag, and detracts from the thrust produced by the bypassed booster flow. The pressure ratio of the booster is higher than that produced by the propeller. Thrust output per unit power input decreases as exhaust pressure is increased. Accordingly, high pressure in the booster bypass stream is a less efficient way of producing thrust with the same power input to the rotor and/or shaft driving the propeller and booster. This is because the propulsive efficiency of the propeller is higher than the propulsive efficiency of the booster and nozzle combination. Therefore, a propeller and booster design is needed that produces a high pressure ratio for that portion of the flow entering the gas generator inlet and low pressure ratio and low airflow for that portion of the flow bypassing the gas generator inlet. Such a configuration is the subject of the present invention.